Method and system for controlling an electromechanically actuated brake for motor vehicles

ABSTRACT

A method and system for controlling a brake for motor vehicles. The present invention relates to a method as well as a system for controlling a brake for motor vehicles, which can be electromechanically actuated by means of an actuator, consisting of an electric motor as well as a drive mechanism positioned downstream of the electric motor. In order to be able to increase motor speed under certain operating conditions, i.e. according to demand, without reducing the available torque, the invention provides for the slope of the speed-torque characteristic curve of the electric motor ( 4 ) to be modified by weakening components of the electromagnetic field of said electric motor ( 4 ), which affect the dynamics of same, in such a way that a higher speed (ω′ 1 ) is obtained at the same torque (M 1 ).

BACKGROUND OF THE INVENTION

The present invention relates to a method and a system for controlling abrake for motor vehicles, which can be electromechanically actuated bymeans of an actuator consisting of an electric motor and a drivemechanism positioned downstream of the electric motor.

The present invention relates to a method and a system for controlling abrake for motor vehicles, which can be electromechanically actuated bymeans of an actuator consisting of an electric motor and a drivemechanism positioned downstream of the electric motor.

A type of method or system for controlling an electromechanicallyactuated brake, for example, has been shown in the German publishedapplication DE 195 36 694 A1. The special characteristic of the systemknown in the prior art is that means are provided, which determine theposition of an actuating element relative to the actuator, that acontroller is provided which can switch between a first and a secondcontrol mode, with the actual position of the actuating element matchinga set value in the first control mode and an actual signal representinga delay matching a desired signal in the second control mode, and that adecision circuit activates the first or the second control modedepending on a decision criterion.

In addition to the available space as well as a defined power input(basically the peak and continuous current should be as low aspossible), above all the following requirements regarding theperformance of the brake need to be taken into consideration whendesigning or dimensioning an electric motor suitable for theaforementioned electromechanically actuated brake (definition of theelectric motor characteristic curve):

1) The distance between friction lining and brake disc (so called freetravel) should be overcome as quickly as possible. Hence, relativelylarge distances have to be covered with a low level of force, whichmeans that the electric motor has to run at as high a speed as possible.

2) Required clamping force and release gradients (high speed at low tomedium torque).

3) Dosing the clamping force. For this purpose, relatively large forcesand short distances are required, which means that the electric motorhas to provide as great a torque as possible.

4) A required maximum clamping force. For the electric motor to bedesigned this means that it has to provide correspondingly high torqueat low speeds.

5) Good reversing performance, which is required for traction controlfunctions (e.g. anti-lock system, electronic driving stability control),also necessitates a correspondingly high motor torque.

These contrary requirements cannot at all or not optimally be fulfilledin the available space for the electric motor with the above-mentionedcontrol known in the prior art

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to propose a method and asystem for controlling an electromechanically actuated brake that allowsthe above-mentioned criteria to be fulfilled. For this purpose, thespeed of the electric motor is to be increased under certain operatingconditions, i.e. depending on demand, without reducing the availabletorque.

This object shall be solved in that the slope of the speed-torquecharacteristic curve of the electric motor is influenced correspondingto the desired actuating force as well as the actuating force gradientor the values representing these signals.

In order to realize the concept of the invention, the slope of thespeed-torque characteristic curve of the electric motor is changed insuch a way the weaknesses of those components of the electromagneticfield that a higher speed is reached at the same torque. Hence, theadvantage that the electric motor can be designed with a higher torqueconstant, meaning that the peak current, e.g. when reversing, and thecontinuous current during stationary clamping operations is reduced.

For this purpose, a torque-producing current value and a field-weakeningcurrent value are calculated, which are transformed into the currents tobe supplied to the electric motor.

The torque-producing current value preferably is calculated byevaluating the desired and actual or present actuating force, whereasthe field-weakening current value is calculated by evaluating the actualstate as well as a desired state of the brake.

According to another advantageous feature of the method, the evaluationof the actuating force is based on force control.

The field-weakening current value is calculated by means of a qualitycriterion, whose input values are actuating force, actuating forcevariation and values that determine the working point in thecharacteristic curve of the electric motor.

A first embodiment of the control system according to the presentinvention that can execute the above-mentioned method is characterizedin that:

a) an actuating force controller is provided, to which the differencebetween a desired and an actual value of the actuating force is suppliedas the input value, with said actuating force controller generating theset value of the torque-producing current on the basis of this;

b) a calculating module is provided, to which the desired and actualvalue of the actuating force, the set value of the actuating forcegradient or a value representing the actuating force gradient, theactual value of the motor position as well as signals representing theactual value and set value of the motor speed are supplied as inputvalues, whereupon said calculating module generates the set value of thefield-weakening current on the basis of these input values;

c) with the set values of both the torque-producing and field-weakeningcurrent as well as the signal representing the actual value of the motorposition being transformed into a signal in a current control, whichrepresents the voltage to be applied to the electric motor.

In advantageous preferred aspects of the first embodiment of the controlsystem according to the present invention, the actuating force gradientas well as signals representing the actual value of the motor speed areadditionally supplied to the actuating force controller.

A second embodiment of the control system according to the presentinvention for executing the above-mentioned method is characterized inthat

a) an actuating force controller is provided, to which the differencebetween a set value and an actual value of the actuating force issupplied as the input value, with said actuating force controllergenerating the set value of the motor speed on the basis of this;

b) the set value of the motor speed is compared to the actual value ofthe motor speed, and the difference reached thereby is supplied to aspeed controller, which generates the set value of the torque-producingcurrent therefrom;

c) a calculating module is provided, to which the desired and the actualvalue of the actuating force, the set value of the actuating forcegradient, the actual value of the motor position as well as the signalsrepresenting the actual value of the motor speed are supplied as inputvalues, with said calculating module generating the set value of thefield-weakening current on the basis of these input values,

d) with the set values of the torque-producing current and thefield-weakening current as well as the signal representing the actualvalue of the motor position being transformed into a signal in anelectronic current control, which represents the voltage to be appliedto the electric motor.

In a third embodiment of the control system according to the presentinvention for executing the above-mentioned method, it is proposed that

a) an actuating force controller be provided, to which the differencebetween a desired and an actual value of the actuating force is suppliedas the input value with said actuating force controller generating afirst set value of the motor speed on the basis of this;

b) a precontrol module be connected in parallel to the actuating forcecontroller, and that the actual value of the actuating force as well asthe set value of the actuating force gradient be supplied to suchprecontrol module as input values, with its output values correspondingto an additional set value of the motor speed as well as an additionalset value of the torque-producing current;

c) the additional set value of the motor speed are added to the firstset value of the motor speed and compared to the actual value of themotor speed, and that the difference arising therefrom is supplied to aspeed controller, which generates the controlled set value of thetorque-producing current on the basis of this;

d) the additional set value of the torque-producing current are added tosaid controlled set value, with the result of the addition being the setvalue of the torque-producing current;

e) a calculating module is provided, to which the desired and the actualvalue of the actuating force, the set value of the actuating forcegradient, the actual value of the motor position as well as the signalsrepresenting the actual value of the motor speed are supplied as inputvalues, with said calculating module generating the set value of thefield-weakening current on the basis of the input values,

f) and the set values of the torque-producing current andfield-weakening current as well as the signal representing the actualvalue of the motor position are transformed into a signal in anelectronic current control, with said signal representing the voltage tobe applied to the electric motor.

Further details, features and advantages of the invention are providedin the following description of an embodiment, with reference to theenclosed drawings. The drawings are as follows:

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of a control system for executing the methodaccording to the present invention;

FIG. 1 is a block diagram of a control system for executing the methodaccording to the present invention;

FIG. 2 shows the configuration of the current control for generating thedesired motor voltage, which is applied in the control system accordingto FIG. 1;

FIG. 3 shows a first embodiment of the actuating force module applied inthe control system according to FIG. 1;

FIG. 4 is a second embodiment of the actuating force module applied inthe control system according to FIG. 1;

FIG. 5 shows the torque-speed characteristic curves of the electricmotor according to FIG. 1; and

FIGS. 6a, b shows the time characteristics of the actuating force, motortorque and motor speed without and with the effect of the measureaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The control system shown in FIG. 1 essentially consists of an actuatingforce control module 1, a calculating module 2 connected in parallel tothe actuating force control module 1 as well as a current control 3arranged downstream of actuating force control module 1 and calculatingmodule 2, whose output signal U_(Mot,Soll), which corresponds to the setvalue of the motor voltage is used to control electric motor 4 (onlyshown schematically) of an electromechanically actuated brake, which ismarked with reference number 5. Electric motor 4 preferably is providedwith a position measuring system 6, whose output signal (ω_(Mot,Ist),which represents the actual position of the motor, is supplied tocurrent control 3 as well as calculating module 2. In addition, theoutput signal (ω_(Mot,Ist) is subjected to time-differentiatingprocessing in a differentiating circuit 8. The output signal ofdifferentiating circuit 8, which corresponds to the actual value of themotor speed (ω_(Mot,Ist), is supplied to calculating module 2 as anadditional input value.

The control system shown in FIG. 1 essentially consists of an actuatingforce control module 1, a calculating module 2 connected in parallel tothe actuating force control module 1 as well as a current control 3arranged downstream of actuating force control module 1 and calculatingmodule 2, whose output signal U_(Mot, Soll), which corresponds to theset value of the motor voltage, is used to control electric motor 4(only shown schematically) of an electromechanically actuated brake,which is marked with reference number 5. Electric motor 4 preferably isprovided with a position measuring system 6, whose output signalÖ_(Mot,Ist), which represents the actual position of the motor, issupplied to current control 3 as well as calculating module 2. Inaddition, the output signal Ö_(Mot,Ist) is subjected totime-differentiating processing in a differentiating circuit 8. Theoutput signal of differentiating circuit 8, which corresponds to theactual value of the motor speed Ù_(Mot,Ist), is supplied to calculatingmodule 2 as an additional input value.

As also shown in FIG. 1, the signals corresponding to the set values ofthe actuating force and the actuating force gradient, the actual valueof the actuating force as well as the set value of the motorspeed—F_(Bet,Soll), {dot over (F)}_(Bet,Soll), F_(Bet,Ist),Ù_(Mot,Soll)—are supplied to the above-mentioned calculating module 2 asadditional input values. It is the task of calculating model 2 togenerate a signal I_(d,Soll) that corresponds to a share of the currentthat, as will be explained in detail below, will influence the slope ofthe torque-speed characteristic curve of the electric motor 4. Theoutput signal of calculating module 2, which is provided to currentcontrol 3 as a second input value, corresponds to the set valueI_(d,Soll) of a share of the current which weakens the components of theelectromagnetic field of the electric motor that change its torque-speedcharacteristic curve. Calculating module 2 preferably exhibits fuzzylogic, which contains decision criteria as to whether thefield-weakening share of the current is to be applied. An example of theapplication could be overcoming the free travel.

The control of the above-mentioned electromechanically actuated brake 5is based on a force control, wherein the set value for the actuatingforce to be set F_(Bet,Soll) is specified by an operating element or asuperior system unit. The set value for the actuating force to be setF_(Bet,Soll) is compared with a signal F_(Bet,Ist) corresponding to theactual value of the actuating force and the ensuing control deviationÄF_(Bet) is supplied to actuating force control module 1. Alternativelya signal {dot over (F)}_(Bet,Soll) corresponding to the actuating forcegradient as well as the above-mentioned signal Ù_(Mot,Ist) correspondingto the actual value of the motor speed can be supplied to actuatingforce control module 1 as additional input values. For this purpose, thesignal F_(Bet,Ist) corresponding to the actual value of the actuatingforce preferably is supplied by force measuring element 7 that is shownonly schematically. It can also be obtained with a method controllingthe electromechanically actuated brake without sensors. The outputsignal of actuating force control module 1, which is made available asan input value to current control 3, corresponds to the set valueI_(q,Soll) of a torque-producing share of the current.

The configuration of current control 3 mentioned in connection with FIG.1 is illustrated in FIG. 2. As shown in the drawing, current control 3essentially is made up of two controllers 9, 10, two transformationelements 11, 12 as well as a function block 13 for calculating therotational voltages of the motor. The first controller 9 is suppliedwith the result ÄI_(q) of a comparison between the set value of thetorque-producing current I_(q,Soll) and the actual value I_(q,Ist),which is provided by the first transformation element 11. The outputvalue U_(q) of the first controller 9 is compared to a first rotationalvoltage U_(q,rot) and the ensuing deviation ÄU_(q) supplied to the firstinput of the second transformation element 12.

The result ÄI_(d) of a comparison between the set value of thefield-weakening current I_(d,Soll) and an actual value I_(d,Ist), whichalso is provided by the first transformation element, is supplied to thesecond controller 10. The output value U_(d) of the second controller 10is compared to a second rotational voltage U_(d,rot) and the ensuingdeviation ÄU_(d) is supplied to the second input of the secondtransformation element 12.

The above-mentioned actual values of the torque-producing currentI_(q,Ist) as well as the field-weakening current I_(d,Ist) are merelymathematical values; they are formed in the first transformation element11 on the basis of the individual phase currents I_(u), I_(v), I_(w) inelectric motor 4 as well as the signal ö corresponding to the motorposition, which represent real values. Also the values ÄU_(q) andÄU_(d), which are supplied to the second transformation element 12, aremerely mathematical values; they are used to form real voltage valuesU₁, U₂, U₃ in the transformation element 12, taking into account themotor position ö. The real voltage values U₁, U₂, U₃ are converted intoset values of the phase voltages U_(u), U_(v), U_(w) to be supplied tothe electric motor corresponding to U_(d,rot, U) _(q,rot).

As already mentioned above, FIG. 3 shows a first embodiment of theactuating force control module 1 described in connection with FIG. 1. Inthe embodiment shown the control deviation ÄF_(Bet) is converted intothe set value of the motor speed Ù_(Mot,Soll) in an actuating forcecontroller 14, with such value being compared to the actual value of themotor speed Ù_(Mot,Ist), and the result of the comparison ÄÙ_(Mot) isconverted into the set value of the torque-producing share of thecurrent I_(q,Soll) in a speed controller 15 positioned downstream.

In the second embodiment of the actuating force control module 1 shownin FIG. 4, a precontrol unit 120 is connected in parallel to theactuating force controller 114. The set value of the actuating forcegradient {dot over (F)}_(Bet,Soll) and the actual value of the actuatingforce F_(Bet,Ist) are supplied to precontrol unit 120 as input values.On the one hand, precontrol unit 120 calculates a set value for themotor speed Ù_(Mot,Soll,Vor), which takes into account the systemrigidity, on the basis of the aforementioned set value of the actuatingforce gradient {dot over (F)}_(Bet,Soll) and, on the other hand, itcalculates the set value of a torque-producing share of the motorcurrent I_(q,Soll,Vor) from the actual value of the actuating forceF_(Bet,Ist) which serves to compensate the influence of disturbances.The set value of the motor speed Ù_(Mot,Soll,Vor), is taken into accountin the formation of the control deviation Ä₁Ù_(Mot), which is convertedinto the set value of a controlled torque-producing share of motorcurrent I_(q,Soll,Reg) in a downstream speed controller 115. Thepreviously mentioned set value of the torque-producing share of motorcurrent I_(q,Soll,Vor) is added to the controlled torque-producing shareof motor current I_(q,Soll,Reg), with the result of the additioncorresponding to the set value of the torque-producing share of motorcurrent I_(q,Soll) supplied to current control 3 (FIG. 1). In thisconnection, the configuration of the controllers 114, 115 mentioned inconnection with FIG. 5 can be identical to the configuration of thecontrollers 14, 15 shown in FIG. 3.

To illustrate more clearly the effect of the field weakening, FIGS. 5and 6 show the motor characteristic curves of the electric motor as wellas typical time characteristics of a clamping procedure of anelectromechanical brake.

In FIG. 5 solid Line I represents the characteristic curve of anelectric motor not applying the method according to the presentinvention, i.e. no field-weakening measures are applied. Theillustration shows that no motor torque is applied yet at idling speedÙ₀. Whereas the motor torque corresponding to the speed value Ù₁ isindicated by M₁, M₀ refers to the (maximum possible) standstill torque.The dotted characteristic curve II shows that the rise of thecharacteristic curve is changed by the effect of the field weakening inthat an essentially higher motor idling speed ù′₀ is reached, so thatalso a higher speed ù′₁ corresponds to the aforementioned torque M₁,without reducing the standstill torque.

FIG. 6a shows the performance of the actuating force F_(Bet), the motorspeed Ù_(Mot) and the motor torque M_(Mot) without field weakening. Ingeneral the motor torque is proportional to the torque-producing shareof motor current and can be derived therefrom by means of the torqueconstants. FIG. 6a illustrates that the aforementioned free travel hasbeen overcome at instant T₁ and the actuating force F_(Bet) begins toincrease. The desired target force is reached at instant T₂. FIG. 6b onthe other hand shows the clamping procedure described above withsuitable control of the electric motor when the aforementioned fieldweakening is executed. If one compares the processes shown in FIG. 6aand 6 b, it becomes evident that the speed ù′₁ corresponding to the samevalue M₁ of the electric motor is considerably higher than the speed ù₁shown in FIG. 6a. Due to the increase in speed achieved through thefield weakening, the time interval 0-T₁ shown in FIG. 6b, which isnecessary to overcome the free travel, is considerably shorter than thetime interval 0-T₁ shown in FIG. 6a. The same holds true for the timeinterval 0-T₂ which is needed to reach the target force.

What is claimed is:
 1. A method for controlling a brake of a motorvehicle actuated electromechanically by an actuator having an electricmotor and a drive mechanism positioned downstream of the electric motor,the electric motor exhibiting a speed-torque characteristic curvedefined by design of the electric motor, the method comprising:receiving a difference value indicative of a value between thedifference of a desired value and an actual value of an actuating force(F_(Bet,Soll)) as an input value; generating a set value (I_(q,Soll)) ofa torque-producing current; receiving the desired and actual values ofthe actuating force (F_(Bet,Soll), F_(Bet,Ist)), a set value of anactuating force gradient (F_(Bet,Soll)), and signals representing anactual value (ω_(Mot,Ist)) of a position of the electric motor and theactual value of the motor speed; generating a set value (I_(d,Soll)) ofa field-weakening current; and converting the set values of thetorque-producing current and the field-weakening current as well as asignal representing a actual value (ω_(Mot,Ist)) of the electric motorposition to a signal (U_(Mot)) that represents a voltage to be appliedto the electric motor, affecting a rise of the speed-torquecharacteristic curve of the electric motor.
 2. The method according toclaim 1 further comprising: calculating set values of a torque-producingcurrent and a field-weakening current; and converting the set values ofthe currents to voltages to be supplied to the electric motor.
 3. Themethod according to claim 2 wherein calculating the set value of thefield-weakening current is based on evaluating an actual state and adesired state of the brake.
 4. The method according to claim 3 whereincalculating the set value of the field-weakening current includescalculating the set value of the field-weakening current based on aquality criterion of which input values include an actuating force, anactuating force variation, and values which determine an operating pointin the characteristic curve of the electric motor.
 5. The methodaccording to claim 2 wherein calculating the set value of thetorque-producing current includes evaluating the desired actuating forceand a current actuating force.
 6. The method according to claim 1further comprising changing the slope of the speed-torque characteristiccurve of the electric motor so that a higher speed is reached at thespeed torque.
 7. A system including an electric motor and a drivemechanism positioned downstream of the electric motor for controlling abrake actuated by an electric motor, the system comprising: an actuatingforce control module (1) which receives a difference value indicative ofa value between the difference of a desired value and an actual value ofan actuating force (F_(Bet,Soll)) as an input value and generates a setvalue (I_(q,Soll)) of a torque-producing current; and a calculatingmodule (2) which receives the desired and actual values of the actuatingforce (F_(Bet,Soll), F_(Bet,Ist)), a set value of an actuating forcegradient (F_(Bet,Soll)), and signals representing an actual value(ω_(Mot,Ist)) of a position of the electric motor and the actual value(ω_(Mot,Ist)) of a motor speed, wherein the calculating module generatesa set value (I_(d,Soll)) of a field-weakening current, wherein the setvalues of the torque-producing current and the field-weakening currentas well as the signal representing the actual value (ω_(Mot,Ist)) of themotor position are converted by an electronic current control (3) to asignal (U_(Mot)) that represents the voltage to be applied to theelectric motor (4).
 8. A system according to claim 7, wherein theactuating force control module (1) receives an actuating force gradientsignal corresponding to the actuating force gradient (F_(Bet,Soll)). 9.A system according to claim 7 wherein the actuating force control module(1) receives an actual value of the motor speed signal corresponding tothe actual value of the motor speed (ω_(Mot,Ist)).
 10. A method ofcontrolling a brake assembly actuated electromechanically by anactuator, the brake assembly including an electric motor and a drivemechanism positioned downstream the electric motor, the methodcomprising: providing an actuating force controller to receive an inputvalue representative of a difference value indicative of a value betweenthe difference of a desired value and an actual value of an actuatingforce and to generate a set value therefrom; providing a pre-controlmodule connected and parallel to the actuating force controller, thepre-controller module being configured to receive the actual value ofthe actuating force and a set value of an actuating force gradient asinput values, the pre-control module being configured to provide anadditional set value of a motor speed and an additional set value of atorque-producing current as corresponding output values; adding theadditional set value of the motor speed to the set value of the motorspeed; supplying an ensuing difference to a speed controller to generatea controlled set value of the torque-producing current therefrom; addingthe additional set value of the torque-producing current; with acalculating module, receiving the desired and actual values of theactuating force, the set value of the actuating force gradient, signalsrepresenting an actual value of a motor position, and an actual value ofthe motor speed; generating a set value of a field-weakening current onthe bases of the values received by the calculating module; andconverting the set values of the torque-producing current, the setvalues of the field-weakening current, and the signals representing theactual value of the motor position to an electronic current controlsignal representing the voltage to be applied to the electric motor. 11.A system including an electric motor and a drive mechanism positioneddownstream of the electric motor for controlling a brake actuatedelectromechanically by means of an actuator, the system comprising: anactuating force controller (14) to receive a difference value (ΔF_(Bet))indicative of a value difference between a desired value and a currentvalue of an actuating force (F_(Bet,Soll), F_(Bet,Ist)), the actuatingforce controller (14) being configured to generate a set value(ω_(Mot,Soll)) of a motor speed and to compare the set value to anactual value (ω_(Mot,Ist)) of the motor speed so that an ensuingdifference (Δω_(Mot)) is supplied to a speed controller (15), whichgenerates a set value (I_(q,Soll)) of a torque-producing current; and acalculating module (2) to receive the desired value and the currentvalue of the actuating force (F_(Bet,Soll), F_(Bet,Ist)), a set value ofan actuating force gradient (F_(Bet,Soll)) from an operating element,signals representing an actual value (ω_(Mot,Ist)) of a motor positionfrom a position measuring system (6) and the actual value (ω_(Mot,Ist))of the motor speed from a force measuring element (7) to generate a setvalue (I_(d,Soll)) of a field-weakening current so that the set values(I_(q,Soll), I_(d,Soll)) of the torque-producing current and thefield-weakening current as well as the signal representing the actualvalue (ω_(Mot,Ist)) of the motor position are converted in an electroniccurrent control (3) to a signal (U_(Mot)) representing voltage to beapplied to an electric motor (4).